Before the demonstration, boil the water that will be used in the flask to make it air-free.

Procedure

a Fill the iron flask with water.

b Remove all air bubbles and screw the plug in.

c Bury the bottle in the freezing mixture and cover it with a cloth. (If solid carbon dioxide is available, the iron flask can be embedded in that.)

Teaching notes

1 The breaking of the flask will be clearly heard. Students are not surprised when glass breaks, but the strong iron flask is another matter.

2 When water freezes it swells. The volume of the ice is about 10% greater than the volume of the water. Water is unusual in expanding when freezing, but it is not the only substance which does so. Iron also expands on freezing and this is why it can be cast.

3 As the water becomes solid, the molecules take up fixed positions in relation to each other. Average spacings between molecules are larger than when the water is in a liquid state.